This invention relates to plastic compositions which are directly electroplatable and to the production of electroplated compositions. More particularly it relates to specific compositions based on hydrocarbon polymers which are capable of being directly electroplated after a simplified sequence of steps, to give highly decorative, well-adhering metal coatings.
The electroplating process for placing metal on plastics has many advantages over other methods including vacuum evaporation and chemical vapor deposition, such as the ability to place heavier metal coatings with good adhesion and good durability.
Conventional preelectroplating processes for plastics have been described often in the literature. A recent outline of such processes is given, for example, in an article by G. K. Schwarz, Industrial Finishing and Surface Coatings (London) 23, 277, pages 12, 14, 26 (1971). In principle, in order to electroplate metals on plastics an electrically conductive surface is required, and this is normally obtained by starting with an electroless plating process. This involves (1) a conditioning step wherein the surface of the plastic is etched or roughened in an acid bath to promote the formation of a strong bond, (2) a sensitizing step wherein the conditioned surface of the plastic is made potentially catalytic to electroless plating by adsorbing precious metal salts such as gold, silver, or palladium chloride, (3) an accelerating step wherein the plastic is immersed in an accelerator solution consisting of a reducing agent such as tin or titanium salts, which results in the formation of free precious metal catalyst on the surface of the plastic, and then (4) electroless plating by dipping the plastic in an electroless plating bath of copper, nickel or cobalt which is chemically reduced on the treated surface until a metal plate of sufficient thickness is deposited to permit conventional electrolytic plating, such as 10-40 millionths of an inch. The precious metal nuclei catalyze the initial copper or nickel deposition from the electroless baths, causing the metals to deposit further on the initial coating by an auto-catalytic mechanism. Careful rinsing steps are essential between the baths to prevent their contamination and/or destruction.
Sensitizing step (2) and accelerating step (3) described above are sometimes reversed in order of application or even combined by using an acid colloidal tin/palladium hydrosol. Many refinements and variations are made in the electroless plating process to meet the requirements of coating various materials.
Following the electroless nickel, cooper or cobalt deposition, the metal coat typically is electrolytically built up with a semi-bright nickel strike, followed by thick ductile acid copper, bright nickel and chromium electroplates. Other combinations and sequences of electrolytic coating can be used.
The complexity and sensitivity of the preelectroplate system and the sensitivity of the plastics to molding conditions have limited commercial growth of the plating-on-plastics field. A recent review of the complexities involved in the catalyzing, sensitizing and activating steps has been written by N. Feldstein in "Plating", June, 1973, pages 611-616.
Acrylonitrile-butadiene-styrene (ABS) resin is the most commonly used plastic for metal plating. However, the cost of plating grades of ABS is rather high, and the molding, surface treating, and electroless plating requirements for quality products are rigorous and add substantially to costs. Also, the properties of plated ABS are not all that might be desired.
Use of polypropylene, rather than ABS, as a polymeric base to be metal plated has certain advantages. Polypropylene is more chemically inert, has much lower water absorption, lower molded-in stresses, and is a lower cost material. However, there are also problems with plating polypropylene, especially in the form of injection molded articles. Mold shrinkage is more pronounced than for ABS, and sink marks or warpage can occur unless special care is taken in mold design. The plating process for polypropylene, starting with electroless plating, is at least as complex as for ABS. Certain special plating grades of polypropylene are available, but they are considerably more expensive than general purpose grades, hence the cost advantage over ABS is largely lost.
Other polymers have also been metal plated, such as polysulfones, polyaryl ethers, modified polyphenylene oxide, glass filled epoxy resins, and glass filled nylon. However, these are all much more expensive raw materials than the ABS and polypropylene discussed above.
Other polymers, additives and fillers have been blended with polypropylene or other plastics to improve etchability or plateability of the unmodified plastic, including talc, metal powders, reducible metal oxides and carbon black. In most cases, a preplate sequence including catalytic activation of the surface and electroless plating of the first metal layer is used.
For instance, U.S. Pat. No. 3,627,576 -- Knorre et al. (1971) concerns the addition to plastic resins of inorganic fillers having functional groups that aid in the binding of catalytic metal coatings used in the electroless plating. U.S. Pat. Nos. 3,694,249 and 3,695,917 -- both Abu-Isa (1972) add organic materials to polypropylene to enhance etching preparatory to catalytic treatment for electroless plating. U.S. Pat. No. 3,562,790 -- Coover et al. (1971) and abandoned U.S. application Ser. No. 677,876 -- Sutherland, filed Oct. 25, 1967, are concerned with tricomponent blends of (a) polypropylene and certain propylene copolymers, (b) polyethylene and certain ethylene copolymers, and (c) certain polydienes to produce compositions having various desirable properties including the ability to serve as a base for adherent metal plates, generally first applied electrolessly.
From the above discussion, it can be seen that it would also be of value to be able to reduce the number of preelectroplating steps, or preferably to eliminate the electroless plating and directly electroplate on economical plastics with good properties. Some steps have been eliminated in some processes, but optimally economical routes to the production and treatment of low cost polypropylene which can be directly electroplated with desired combinations of decorative finish and mechanical properties and without prior electroless plating are not generally known.
Other prior art provides conductive paints that can be applied to a variety of substrates to facilitate direct electroplating, such as the carbon-black-filled ABS coating of Japanese Patent Publication 46-16,437 -- Ohno, published May 6, 1971. Adhesion between the paint and the substrate can be a problem in such systems.
Concerning conductive polymeric blends, conductive or antistatic vulcanized rubbers and plastics containing large quantities of graphite, carbon blacks or powdered metals have been known for many years, chiefly for the dissipation of electrostatic charges in cables, tires, belts, hospital equipment, etc. They have also been used in heating elements, resistors and radio-frequency applications. Such applications are reviewed by R. H. Norman, "Conductive Rubbers & Plastics", Elsevier Publishing Co. (New York) (1970) pages 223-258. Also, direct copper electroplating of conductive vulcanized rubber to provide electrodes for measuring conductivity has been described by R. G. Newton, Journal of Rubber Research 15, 35-60 No. 3 (March, 1946). High contact resistance on the surface is characteristic of these compositions when molded due to a thin skin of unfilled polymer. It is known that certain high-structure carbon blacks such as "Vulcan" XC-72 produced by Cabot Corp. of Boston, Massachusetts can be blended with various rubbers such as terpolymers of ethylene, propylene and dienes, to produce highly conductive rubbers. Being elastomeric, these products lack the rigidity desired in many plastic parts, especially those to be electroplated.
Uebigau, in Kuntstoffe, Vol. 49, pages 45-47 (January, 1959), describes filling phenolic molding powder with about 50 percent by weight of conductive carbon black or graphite. The surface is then roughened by sand blasting, concentrated alkali, or anodic oxidation. After degreasing and acidification, the moldings were directly copper-plated. However, the surfaces generally require post-treatments with a polishing step, using a polishing wheel or electropolishing.
Others have described similar materials which are characterized by various problems. Canadian Patent 883,425 to Nebel et al. (1971) discloses that it was previously known that resins could be metallized directly if they contained high levels of electrically conducting materials (graphite), but it notes that this impairs the mechanical strength of the resin. Also, U.S. Defensive Publication T904,012 -- Staniland et al. (1972) discusses the use of up to 50 percent carbon black filler and optionally metal powders for permitting electroplatability of certain aromatic polymers including particularly polysulfones.
It is known that tensile strength of vulcanized rubber formulations can be increased by replacing part of fine particle-sized furnace carbon black with fine particle-sized platey talc filler, compared to the use of either filler alone. Also, mill shrinkage, modulus value and viscosity are reduced, and elongation is increased. Brochure 3R "United Sierra Mistron Vapor in Rubber Compounds", pages 65-69, published in about 1967 by United Sierra, Division of Cyprus Mines Corp. However, this publication does not discuss effects on electrical conductivity.
Previously published work with polyethylene filled with carbon black showed that the addition of 1% finely divided silica "Aerosil A-175" increased conductivity slightly but that 3 to 5% additions caused decreases in conductivity. Yu. I Vasilenok et al. 77 Chem. Abs. 153007 (1972) published as Plastycheskii Massi 1972, No. 8, p. 35, and in translation in Soviet Plastics 1972, No. 8, 39-42 by Rubber and Plastics Research Association of Great Britain.
Also, Norman, supra, page 59, indicates that the addition of various non-conductive fillers in natural rubber plays no part in determining the resistivity which is determined only by the relative quantity of carbon black in the total compound.
Japanese Patent Publication 14,188/73 -- Kobayashi, published May 4, 1973, discloses the direct electroplating of a composite blend in certain proportions of thermoplastic resin, rubber which can be etched by acid, and special types of carbon black. Examples are given with the thermoplastic being polypropylene and polyvinyl chloride, and it is also disclosed that polystyrene and polyethylene can be used. The rubber is said to aid the blending of the thermoplastic and the carbon black. The carbon blacks to be used are highly conductive, high structure blacks with an oil absorption number of 400 to 600 cc./100 g. Such high structure blacks are generally quite expensive and can increase the problems of processing. Good mechanical properties are said to result. Although these compositions can be desirable, it would be better if blends could be discovered that would not require such special carbon blacks and which would have even better processability and decorative and mechanical characteristics than these materials.
In summary, there is still a need for low cost, polypropylene-based compositions which can be formed and directly electroplated by relatively simple procedures to give plated products with optimum appearance and mechanical properties in combination.